The present disclosure is related to solid state laser devices, and more specifically to ridge waveguide semiconductor lasers.
Modern semiconductor lasers, such as that shown in FIG. 5, are typically a series of layers comprising: a lower contact 102, a substrate and lower cladding 104, a lower waveguide layer 106, an active layer 108, an upper waveguide layer 110, an upper cladding layer 112, and an upper contact 114 which is separated in part from the upper cladding layer 112 by patterned insulator 116. Lateral end faces (in a plane generally perpendicular to the plane of the active layer 108) are formed to have semi-reflective mirror-like surfaces. Light is generated within the active layer, which is reflected between the semi-reflective mirrored end faces to ultimately produce a coherent laser output L which exits through at least one of the end faces.
In order to confine the light within the optical cavity of the structure so that stimulated emission can take place, the materials of the cladding layers 104, 112 are chosen to have an index of refraction which is lower than the index of refraction of the materials forming waveguide layers 106, 110 and active layer 108. Thus, the interfaces of the waveguide layers and the cladding layers are essentially mirrors, confining the light to the active layer. This is referred to as transverse confinement, since it confines the light in a direction transverse to the optical axis, i.e., the axis of the light beam ultimately emitted by the structure.
To confine the light within the structure in a lateral direction, i.e., within the plane of the active layer but still perpendicular to the optical axis of the device, a relative lowering of the refractive index (index guiding) at the lateral boundaries of the semiconductor laser structure may be provided. The lowering of the refractive index can be achieved either by etching the outer part of the semiconductor layers or by inducing an index change of the outer semiconductor material (e.g., implantation-induced disorder of quantum wells).
In one class of semiconductor light-emitting devices, referred to as ridge waveguide lasers, the lateral dimensions of parts of the upper layers 110, 112, and 114 are limited, either physically or functionally, for example by way of patterning and etching. Additionally, an underlayer, such as patterned underlying insulator 116 with a lower refractive index than semiconductor layers 106-112 may be used to confine the electrical injection to the so defined portion of the semiconductor structure. In certain embodiments such as shown in FIG. 5, when viewed in cross section this appears as a ridge (or stripe) 118 of width w running parallel to the optical axis. In the plane of the active region, light generation is thereby confined in what is referred to as the injection region by index guiding (i.e., the refractive index of the injection region, e.g. below the upper contact ridge 118, is larger than in adjacent regions below underlying insulator layer 116).
Recently, various non-epitaxial, conductive materials have been explored for use as the upper cladding layer. The problems being addressed by these non-traditional, conductive materials include that certain combinations of semiconductor waveguide layer materials 110 and cladding layer materials 112 have relatively low refractive index differences, are difficult to grow epitaxially upon each other without adversely affecting active layer quality and device performance, and/or are difficult to make electrically conductive. Silver, in particular, has been suggested as a material of interest for the upper cladding layer. See, for example, U.S. patent application Ser. No. 12/237,106, filed Sep. 24, 2008, which is incorporated herein by reference.
Conductive materials in general contribute to losses in semiconductor lasers by way of free carrier absorption mechanisms. In order to minimize such losses the penetration of the optical wave into a conductive cladding layer must be minimized. Therefore, the refractive index difference between the conductive cladding layer material and the waveguide material 110 needs to be as large as possible, e.g., a very low real part of the refractive index of the cladding layer is required. Known conductive materials having a very low real part of the refractive index in the visible spectrum (or portions thereof) of are for example silver (Ag), gold (Au), and aluminum (Al).
One problem still to be addressed is use of the non-epitaxial conductive cladding layer without an intervening reflective layer so that it may act as both a lateral waveguide and ohmic contact. The real portion of the refractive index of silver, for example, is lower than that of most other materials and in particular lower than that of air and other commonly used insulating materials (SiO2, Si3N4 etc.) in the ultraviolet (UV) to near-infrared (IR) spectral range. However, lateral mode guiding occurs at the location of the laterally highest effective refractive index, which would not be below the contact structure in the case of a uniform non-epitaxial conductive cladding layer. The effective refractive index is given by the weighted sum of the refractive indices of all constituent materials including air. The relative contributions to the effective refractive index are defined by the penetration depth of the light into each material as seen from the semiconductor laser structure. Since the same applies to the effective absorption in the contact layers, the absorption of any material involved must be low at the wavelength of interest.
A non-epitaxially, conductive cladding layer with refractive index lower than air and other insulating materials formed to have a traditional ridged structure cannot, therefore, provide adequate optical confinement over the injection region. Thus, there is a need in the art for a technique to provide a silver cladding layer with an increased index of refraction so that it may function both as a refractive index waveguide and as an ohmic contact for edge-emitting laser diodes and similar devices, with minimal specialized processing requirements.